1. Field of the Invention
The invention relates to a push-button switch device for a multifrequency dialling telephone set.
A push-button switch for multifrequency dialling basically comprises a set of push-buttons, usually twelve, a multifrequency generator, means for selecting groups of frequencies, (usually pairs of frequencies) transmitted by the multifrequency generator for each push-button and means for connecting the multifrequency generator to the telephone line.
2. Description of the Prior Art.
Push-button switches having frequency selector switch for selecting pairs of frequencies respectively associated with push-buttons and a common switch for the telephone line are known in the art. For example, the article by R. ARAKI, K. TAJIMA and H. NAKAZAWA in the Japanese periodical "Review of the Electrical Communication Laboratory", volume 17, No. 12, December 1969, Pages 1437 - 1455, discloses a push-button switch, in which the common switch is connected to the telephone line after one of the frequency selector switches is connected to the multifrequency generator by a pushing force exerted on the associated push-button.
However, the selector switches and the common switch comprise flexible reeds similar to relay reeds. One end of a flexible reed is fixed at the frame of the push-button switch whereas the other end is restored to the inoperative position in which the selected push-button is not pushed, by one or more springs flanged to their bases. Owing to the additional flexural strenghtes exerted by the reeds in the opposite direction to the pushing force exerted on the selected push-button, a relatively high pressure (at least 200 g.) is required, to the user's discomfort. In addition, the displacement amount of the movable ends of flexible reeds limits the stroke of the contacts of the common switch and the transmission duration of the frequency groups to the telephone line. Consequently, this duration is difficult to adjust accurately and the displacement amount of the released ends of the flexible reeds is less than the stroke of the push-buttons.
Other push-button switches have a switch device for connecting and disconnecting an electronic circuit, e.g. comprising a multifrequency generator, and comprise contact bridges rotating on a pivot and, restored by mechanical means associated with respective push-buttons. A contact bridge and a push-button are restored by additional spring forces in opposition to the pressure exerted on the push-button. For example, the number of additional forces is equal to three as described in U.S. Pat. No. 3,731,030. However, additional forces are exerted against an intermediate element indistinguishable from push-button, so that the push-button dimensions cannot be modified to suit the user, and furthermore the contact bridge closes the electronic circuit only when the push-button is completely pushed, i.e. at the end of the push-button stroke. Thus, the transmission or connection duration depends in inconvenient manner on the time during which the push-button is kept pushed down by the user.